Fluoropolymers, or fluorine-containing polymers, are a commercially important class of materials. Fluoropolymers include, for example, crosslinked fluorocarbon elastomers and semi-crystalline or glassy fluorocarbon plastics. Fluorocarbon plastics (or fluoroplastics) are generally of high thermal stability and are particularly useful at high temperatures. They also exhibit extreme toughness and flexibility at very low temperatures. Many of these fluoroplastics are almost totally insoluble in a wide variety of solvents and are generally chemically resistant. Some have extremely low dielectric loss and high dielectric strength, and many have unique nonadhesive and low-friction properties. See, for example, F. W. Billmeyer, Textbook of Polymer Science, 3.sup.rd ed., pp. 398-403, John Wiley & Sons, New York (1984).
Fluorocarbon elastomers, particularly the copolymers of vinylidene fluoride with other ethylenically unsaturated halogenated monomers, such as hexafluoropropene, have particular utility in high temperature applications, such as seals, gaskets, and linings. See, for example, R. A. Brullo, "Fluoroelastomer Rubber for Automotive Applications," Automotive Elastomer & Design, June 1985, "Fluoroelastomer Seal Up Automotive Future," Materials Engineering, October 1988, and W. M. Grootaert, et al., "Fluorocarbon Elastomers," Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 8, pp. 990-1005 (4.sup.th ed., John Wiley & Sons, 1993).
Fluoroplastics, particularly polychlorotrifluoroethylene, polytetrafluoroethylene, copolymers of tetrafluoroethylene, hexafluoropropylene, perfluoropropyl vinyl ether, and poly(vinylidene fluoride), have numerous electrical, mechanical, and chemical applications. Fluoroplastics are useful, for example, in wire coatings, electrical components, seals, solid and lined pipes, and piezoelectric detectors. See, for example, "Organic Fluorine Compounds," Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 11, pp., 20, 21, 32, 33, 40, 41, 50, 52, 62, 70, 71 (John Wiley & Sons, 1980).
In the automotive industry, for example, increased concern with evaporative fuel standards has led to the need for fuel system components that have improved barrier properties. This helps reduce the permeation of fuel vapors through automotive elements such as fuel filler lines, fuel supply lines, fuel tanks, and other elements of an automobile fuel system. Multi-layer tubing and other articles containing a fluorinated layer have been used in such automotive elements to provide a chemically resistant permeation barrier. Multi-layer articles are also useful in a number of other industries including, for example, the chemical processing and/or handling industries, and the electrical and electronics industries. Such multi-layer articles can include one or more other layers that can add strength, rigidity, or other mechanical properties.
Multi-layer compositions comprising a fluorinated polymer layer and a polyamide or polyolefin layer are known. See, for example, U.S. Pat. No. 4,933,090 (Krevor) which discloses laminate tubular articles that can include layers of fluorocarbon elastomers, and International Publication No. WO 93/1493 (LaCourt) which discloses a laminar film structure that includes a polyimide and a fluoropolymer.
To be useful, these multi-layer articles should not delaminate during use. That is, the adhesive bond strength between the layers of the multi-layer article should be sufficient to prevent the layers from separating. A variety of methods have been employed to increase the bond strength between a layer comprising a fluoropolymer and a layer comprising a substantially non-fluorinated polymer. For example, a layer of adhesive can be added between the two layers. However, the adhesive used must not limit the performance of the multi-layer article.
As an alternative to, or in addition to, adhesives, surface treatment of one or both of the layers has been used to increase the adhesive bond strength between the layers. For example, layers comprising a fluoropolymer have been treated with a charged gaseous atmosphere followed by application of a layer of thermoplastic polyamide. Such surface treatments add additional steps and cost to the manufacturing process and are limited to non-coextrusion processes.
In another approach, the adhesion between a substantially non-fluorinated polymer and a fluoropolymer, wherein the fluoropolymer is derived from vinylidene fluoride (VDF), and optionally hexafluoropropylene (HFP), has been found to increase upon exposure of the fluoropolymer to an amine compound. An example includes providing a fluoropolymer comprising interpolymerized units derived from vinylidene fluoride, a layer of a melt-processable, substantially non-fluorinated polymer, and a melt-processable aliphatic di- or polyamine of less than 1,000 molecular weight.
In contrast, fluoropolymers derived from fluorinated monomers that include substantially no VDF are known to be more chemically inert than fluoropolymers derived from VDF monomers, and are more resistant to chemical attack. Thus, such fluoropolymers are ideal for use in composite applications (e.g., articles having multi-layers) where a more resistant barrier layer is desired, such as automotive hose applications. Such articles combine the chemical resistance of the fluoropolymer with the structural properties of a generally thicker and lower cost hydrocarbon material. Examples of such substantially non-VDF derived fluoropolymers include fluoropolymers derived from monomers of tetrafluoroethylene (TFE), hexafluoropropylene (HFP), chlorotrifluoroethylene (CTFE), and optional non-fluorinated monomers. The chemical resistance provided by these fluoropolymers make such composite articles useful as automotive fuel lines, fuel tanks, other elements of automobile systems, as well as liners, tubing and containers in chemical processing and any other use where chemically resistant barriers are desired.
However, because of the improved chemical resistance of these substantially non-VDF derived fluoropolymers, they are also less likely to undergo adhesion-promoting reactions with amines. Although some degree of adhesion may be obtained on exposure of a substantially non-VDF containing fluoropolymer to an amine, many applications will benefit from, and may require, higher adhesion to a fluoropolymer that provides a chemically resistant barrier. Thus, poor adhesion between the non-VDF containing fluoropolymer and a hydrocarbon material makes formation of useful composite articles difficult.
What is yet needed is a composite article that includes a barrier comprising a fluoropolymer that provides improved adhesion to a substantially non-fluorinated polymeric substrate.